This invention relates to a hydraulic autotensioner unit for adjusting the tension in a belt, especially a belt for driving auxiliary equipment such as an alternator, for an engine.
U.S. Pat. No. 4,790,801 and Unexamined Japanese PCT Publication No. 2-500210 disclose hydraulic autotensioner units comprising an arm having one end pivotally mounted on an engine block, a tension pulley rotatably supported on the free end of the arm, and an actuator coupled to the arm to bias it in a direction to stretch the belt, thus keeping constant the tension of the belt.
The actuator mounted in this type of hydraulic autotensioner has a cylinder, a rod inserted in the cylinder, a spring for creating a biasing force that causes the rod to protrude from the cylinder, and a hydraulic damper mechanism mounted in the cylinder for damping the movement of the rod when it is pushed into the cylinder. The spring and the hydraulic damper mechanism cooperate to reduce any variation in tension in the belt running around the tension pulley, thus keeping the tension in the belt constant.
Such a conventional hydraulic autotensioner unit has spring housed in the cylinder, so that its size is limited. In this arrangement, it is impossible to use a spring having a large coil diameter or having a low spring constant. Consequently, the application of a sufficiently high tension to the belt or a minimization of variations in the tension of the belt also become impossible.
One way to use a spring with either a large spring constant or a large coil diameter is to provide such a spring outside the cylinder. But the spring provided outside the cylinder will complicate the mechanism for coupling the actuator to the arm and make assembling of the actuator difficult.
An object of this invention is to provide a hydraulic autotensioner unit having an actuator which can use a spring that has a low spring constant but still is capable of applying a sufficiently high tension to the belt, and which can be assembled easily.